Passive Source Seismic Studies

Natural source seismic studies will provide independent data on crustal anisotropy and thickness structure as well as be the primary data to constrain large-scale mantle structure, anisotropy, and composition. We will utilize the standard techniques of traveltime tomography, receiver functions, surface wave dispersion, and shear-wave splitting studies. In addition, we will apply a relatively new method of estimating layered anisotropy structure from waveform modeling and inversion. Anisotropy structures in both the crust and upper mantle may play a pivotal role in distinguishing between the absence or presence of an eclogitic root, and in the case of delamination, constraining whether the delamination event occurred shortly after formation or is a much younger event

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Research Plan for Natural Source Seismic Studies

The natural source deployment is designed to provide sufficiently high resolution images of the mantle structure to provide direct evidence regarding convective removal of the root. To accomplish this goal, a dense deployment (8 km station spacing) of PASSCAL broadband seismometers is proposed along the two primary transects. Each transect will consist of 25 seismometers deployed along a 200 km long line (Figure 12). This configuration is chosen to optimize the trade-off between the necessary spatial resolution (determined by the station spacing) and the line length needed to image to ~200 km depth.

The following images will be produced beneath the two transects: (1) tomographic images from the surface to 200 km depth of the compressional and shear wave velocity structure derived from inversion of teleseismic body wave travel-times (Humphreys and Dueker, 1994; Dueker et al., 2001); (2) images of the crust and mantle layering down to 200 km depth from migration of converted P to S-wave arrivals (Dueker and Sheehan, 1997; Dueker et al., 2001; Morozov and Dueker, in review); (3) anisotropic lithospheric layering from global minimization inversion of receiver function waveforms (Frederiksen et al., 2002; Folsom et al., 2002), and (4) maps of the mantle anisotropy from measurement of SKS splitting (Schutt et al., 1997). In addition, in the area between the two lines, a unique opportunity is provided to make a high quality tomographic image of the crustal shear wave velocity structure by inversion of surface wave phase velocities measured between the two lines (Baumont et al., 2002). The University of Wyoming (Dueker) will have primary responsibility for the tomographic and receiver function imaging and the University of Arizona (Zandt) will have primary responsibility for the anisotropy and surface wave studies, although we will work together to integrate the natural source seismological results. With these images and accompanying data, we will be able to constrain the following important questions:

· Where is the “residue” from which the plutons of the batholith were extracted? If it delaminated in the past 10 Ma we might be lucky enough to catch a snapshot of the delamination event similar to the one under the Southern Sierras (Ruppert et al., 1998). If a negatively buoyant lower crust did detach (Jull and Kelemen, 2001) and sink into the mantle shortly after formation, then all previous layering associated with the creation, suturing and compression of the CPC crust would be erased. Thus, we would expect truncation of mantle layering. Of particular interest in this respect is the structure beneath the sharp magmatic front where we speculate that the delamination event may have nucleated. However, if the root did not delaminate, but instead remains hidden beneath the CPC’s seismic Moho, then continuity of mantle layering is expected, but the high density of this garnet rich root should produce an observable gravity anomaly. An example of the quality of the sub-crustal layering image we expect from the proposed passive seismic experiment is shown in Figure 16.

· How strongly coupled were the crust and mantle in accommodating deformation? The crustal and mantle anisotropy, as delineated via waveform modeling and shear wave splitting, may provide valuable clues because the emplacement and unroofing of a giant batholith, and possible delamination of a residual root are profound strain events that would leave its marks in the fabrics of the rocks. Advances in processing and modeling of receiver functions, in conjunction with the nearly 3-D active source experiments, present an opportunity to incorporate anisotropy studies to a greater degree than ever before. Note that multiple layers of anisotropy (some layers as little as 5 km thick) are resolvable within the thick crust. An interesting result for this station is that the orientation of the fast direction in the crust is nearly orthogonal to the fast direction in the mantle as measured from SKSsplitting studies, suggesting a decoupling in the strain fields between the crust and mantle. Interpretation of a single station or widely spaced stations is always fraught with large uncertainties; however, in our proposed BATHOLITHS experiment, the density of stations and availability of auxiliary data from the active seismic experiments, as well as constraints from the field structural studies, should provide an opportunity to image the seismic anisotropy structure of the lithosphere to an unprecedented resolution.

· Is the present day lithospheric structure of the CPC most consistent with a preserved or delaminated batholithic root? By combining a detailed crustal seismic model from the active source studies, with the upper mantle images from the natural source studies, we will determine the present day lithospheric structure with significantly better resolution than currently available for either the Southern Sierras (Ruppert et al., 1998) or the Central Andes (Beck and Zandt, 2002). The Sierra Nevada and Central Andes are among the best comparative study areas for the deep structures of an exposed thick, granitic batholith and an active, batholith-forming continental arc, respectively. The Sierra Nevada provide the best analog for a young (<10 Ma) batholithic root delamination event and its geophysical signatures. The Central Andes are perhaps the closest analog of the active continental subduction environment in which the CPC formed. In the Central Andes we found a complex mantle structure with preserved lithospheric mantle beneath the active arc (indicating an intact batholithic root) but evidence for ongoing piecemeal delamination in the back-arc region (Beck and Zandt, 2002). Large but localized silicic volcanic complexes within the back-arc region signal magmatic processes extend well beyond the line of active andesitic volcanoes (Zandt et al., 2002) and are characterized by a local extensional stress regime that can be ascertained by seismic anisotropy studies (Leidig and Zandt, 2002). Comparison of our results from BATHOLITHS to studies from these and other analog regions will play an important role in the synthesis of results from this interdisciplinary effort.

 

Logistics Plan for Natural Source Seismic Studies

The natural source experiment will operate two lines of 25 sites each along the northern and southern transects for 14 months, from June of 2005 to September of 2006. An eight-kilometer spacing between stations will allow two 220 km long line arrays to be operated simultaneously. The westernmost 8 stations on the northern transect and the westernmost 10 stations on the southern transect will require boat or helicopter access; the rest of the 34 stations will be accessible via established roads. Ken Dueker and George Zandt have extensive experience with the operation of PASSCAL broadband seismic experiments in North America and South America . Ken Dueker has operated over 300 PASSCAL broadband sites to date and just completed the operation of 81 PASSCAL sites in the Colorado and Montana mountains during the winter of 2000 with only a few power-related problems. George Zandt has been lead P.I on two broadband seismic experiments and participated in two other deployments in the Andes of South America, as well as involved in several smaller experiments in the western U.S. The University of Wyoming will take the lead in the field deployment and handle the logistics dealing with the shipping and installation of all the instruments, as well as the archiving of the data. The University of Arizona will provide assistance, as needed, in the initial deployment as well as for the service runs.

Keeping the 20 maritime climate coastal sites powered during the winter will require 140 watts of solar panels and 300 amp-hours of batteries along with eight foot high solar panel mast stands. To keep the seismometer vault from flooding, we will use our standard watertight vault design with a large (two cubic feet) poured cement sensor pad to keep the seismometers from tilting out of center. Our experience shows that a somewhat over-engineered installation of the seismometers is cost and data effective.